Between the Middle Ages and modern times, doctors have learned a lot of new things about how human bodies work. The Islamic doctor Al-Tabari, in the early 800s, learned that light bounces off objects into your eyes. Al-Tabari’s student Al Razi figured out that a fever is your body’s way of fighting illness, not part of the illness. By about 1000 AD, Ibn Sina knew that the heart pushed the blood through the body, and that nerves told the muscles to contract or lengthen. He thought different parts of the brain had different functions.
Ibn Sina was still following Galen in thinking that the liver made blood, and possibly other “dense humors” for the body. Ibn al-Nafis, in the 1200s AD, was the first doctor to understand that blood goes from your heart to your lungs to get air, and then back to your heart again to get pumped all over your body. Ibn al-Quff, about the same time, figured out how the blood gets from the arteries to the veins through tiny invisible blood vessels called capillaries.
Leonardo da Vinci, about 1550 AD, used dissection to identify more nerves going from the brain to the eyes, the ears, and the nose. Leonardo also figured out that a man’s penis filled with blood to cause erections. In 1628, William Harvey, in England, confirmed the work of Ibn al-Nafis and Ibn al-Quff with his own experiments. By this time, European missionaries were bringing news of these discoveries directly to China by boat.
A generation later, glass-workers invented microscopes. The Italian scientist Marcello Malpighi used a microscope to confirm the existence of capillaries. Malpighi also used the microscope to see how air got into the blood in the lungs. He was able to see the cell layer that made Africans’ skin black. The Dutch researcher Renier De Graaf used the microscope to see human egg follicles.
About 1650, thanks to dissection and microscopes, British doctors were able to see that the liver really didn’t make blood. But scientists still really didn’t understand the liver. About the same time, Kepler accepted that the eyes really did work like a camera obscura. They projected an upside-down image onto the retina.
A hundred years after the invention of the microscope, in the late 1700s, scientists also figured out more about what electricity was. This new understanding of electricity helped to show that the brain and the nerves used electric impulses to carry messages. Luigi Galvani’s experiments with electricity and frog nerves in 1791 showed the relationship.
Since 1800 AD, other new inventions have helped scientists to figure out more and more about how human bodies work. First, better microscopes let Henri Dutrochet see that people’s bodies were made of cells. Then gradually through the 1800s European and American scientists figured out that cells themselves were made of smaller parts. They found out what lysosomes and mitochondria did.
Even in the 1850s, though, people still had no idea what your liver was for. Then the invention of modern chemistry allowed a French researcher, Claude Bernard, to see that the liver regulated sugar levels, and then over the next generation other researchers gradually figured out that the liver filters poisons out of your blood. About 1900, the Russian researcher Ivan Pavlov figured out a way to observe the insides of living animals, which helped to show how they worked.
About 1900, Wilhelm Röntgen figured out how to take x-rays of the human body. In the 1950s, James Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin used these x-rays to figure out that cell nuclei were made of DNA molecules. In the 1970s, the invention of computers allowed doctors to do CAT scans. These scans showed images of the inside of living bodies. They led to advances in our understanding of how brains work. There are still lots of things we don’t understand about the human body. Scientists are still arguing over whether electricity or chemicals carry messages through nerves. And they aren’t sure whether different parts of the brain control different actions, or whether the whole brain works together as a network.